1. Field of the Invention
This invention relates to novel polyvinyl alcohol compositions. More particularly, this invention relates to solutions of polyvinyl alcohol in mixtures of non-aqueous solvents. Hydrogels prepared from these compositions exhibit improved properties relative to gels prepared from the aqueous or non-aqueous polyvinyl alcohol compositions of the prior art.
2. Description of the Prior Art
Gels, fibers and other products prepared from solutions of polyvinyl alcohol in water or a mixture of water and a water-miscible organic solvent such as methanol, propanol, dimethyl sulfoxide and N,N-dimethylformamide are well known from numerous patents and other publications. Typical of recently issued patents describing gels formed from aqueous solutions of polyvinyl alcohol is U.S. Pat. No. 4,663,358, which issued to Hyon et al on May 5, 1987. This patent teaches cooling to below room temperature a solution of polyvinyl alcohol in a mixture of water and a water-miscible organic solvent such as alcohols containing from 1 to 4 carbon atoms, glycol and dimethyl sulfoxide. Mixtures of water and dimethyl sulfoxide are preferred, and the water can constitute from 10 to 90 weight percent of the solvent mixture. The gel formed by cooling the solution is then immersed in flowing water to remove the organic solvent. Alternatively the initial gel is dried to remove substantially all of the solvent and then immersed in water to form the hydrogel.
Hydrogel prepared as described in the Hyon patent are transparent whereas gels formed using a solution of polyvinyl alcohol in either water or dimethyl sulfoxide as the only solvent are opaque.
Fibers formed from polyvinyl alcohol and various methods for preparing such fibers are discussed in a text by Ichiro Sakurada entitled "Polyvinyl Alcohol Fibers" (Marcel Decker, 1985). Most of the methods for preparing polyvinyl alcohol fibers that are described in this text use a solution of the polymer in either water or a mixture of water and a watermiscible organic solvent. The one notable exception was originally reported by H. M. Zwick and involves using a solvent or solvent mixture that causes phase separation into a solvent and a polymer-rich phase at a point between the spinneret and the take-up reel. The resultant fibers were washed to remove the solvent and then hot drawn using a draw ratio of between 2.6:1 and 8:1. Single solvents suggested for us in this method included benzenesulfonamide, caprolactam, and trimethylolpropane. Solvent mixtures included dimethyl sulfoxide/pentaerythritol and water in combination with urea or thiourea.
Various solvents for use in spinning polyvinyl alcohol fibers are taught in U.S. Pat. Nos. 4,698,194 and 4,603,083, which issued to Tanaka et al. on Oct. 6, 1987; U.S. Pat. Nos. 4,713,290 and 4,599,267, which issued to Kwon et al. on Dec. 15, 1987; and U.S. Pat No. 4,765,937, which issued to Hyon et al. on Aug. 23, 1988.
The Tanaka et al. patents disclose fibers with tenacities greater than 18 grams per denier prepared from solutions of high molecular weight polyvinyl alcohol exhibiting a degree of polymerization of at least 1500 in organic liquids such as dimethyl sulfoxide, glycerin, ethylene glycol, diethylene triamine, ethylene diamine, phenol and aqueous solutions of inorganic salts such as zinc chloride, sodium thioxyanate, calcium chloride, and aluminum chloride. Dimethyl sulfoxide is the preferred solvent. The fibers cannot be spun using the wet-or dry-spinning method that are conventionally used for polyvinyl alcohol fibers, but rather must be prepared using a unique "dry-jet wet spinning" process described in the patent.
The Kwon et al. patents describe fibers with tenacity values greater than 10 grams per denier prepared from solutions comprising from 2 to 15 percent by weight of an ultra-high molecular weight polyvinyl alcohol exhibiting a molecular weight of at least 50,000 (equivalent to a degree of polymerization of at least 1100), preferably between 1,500,000 and 2,500,000, and a solvent.
Suitable solvents have vapor pressures of at least four-fifths of an atmosphere (80 kPa) at 180 degrees C., and include aliphatic and aromatic alcohols of the desired nonvolatility, water and aqueous solutions of salts such as lithium chloride, calcium chloride or "other materials capable of disrupting hydrogen bonds and thereby increasing the solubility of the polymer".
Fibers are formed by extruding a heated polymer solution through at least one orifice and cooling it sufficiently rapidly to form a gel fiber, extracting the solvent used to initially dissolve the polymer with a volatile second solvent, and subsequently drying the fiber to form a "xerogel". The fiber is drawn in one or two stages at any point during or following cooling of the initially extruded fiber.
The Hyon patent discloses polyvinyl alcohol fibers with tenacity values greater than 15 grams per denier. The degree of polymerization of the polymer used to prepare the fibers is preferably greater than 1700, and the polymer is dissolved in a mixture of water and a water-miscible organic solvent. Suitable organic solvents are disclosed in the aforementioned patent to Hyon et al. directed to polyvinyl alcohol gels. Any of the conventional wet- or dry spinning methods can be used to form the fiber.
The fiber is coagulated at a temperature preferably below -20 degrees C. After being dried the fiber is drawn at a temperature of from 140 to 220 degrees C. in air, an inert gas, silicone oil or polyethylene glycol using a draw ratio of from 20 to 200.
The use of boric acid or borate salts as crosslinking agents for polyvinyl alcohol fibers is taught in U.S. Pat. No. 4,612,157. The purpose of the external crosslinking agent is to improve the physical properties of the final fiber. The fibers are prepared by extruding the polymer solution into an alkaline coagulating bath.
The present inventors discovered that by replacing the aqueous solvents of the prior art with mixtures of dimethyl sulfoxide and certain monohydric aliphatic alcohols in proportions specified hereinafter the resultant solutions can be processed to yield useful products, including hydrogels and fibers. The hydrogels exhibit superior properties relative to products prepared using aqueous solutions of polyvinyl alcohol of comparable molecular weight.